Soil: understanding pH and testing soil
When designing and planting your garden, you need to know whether the soil is acid or alkaline, as different plants thrive in different soils. The soil pH is a number that describes how acid or alkaline your soil is. A pH of 7.0 is considered neutral. An acid soil has a pH value below 7.0 and above 7.0 the soil is alkaline.
Timing Test at any time
When to test soil pH
It is especially worth checking soil pH before designing or planting a new garden, making vegetable plots, planting fruit, when growth is disappointing, or where yellowing of foliage occurs.
Testing can be done at any time, but if carried out within three months of adding lime, fertiliser or organic matter, the test may give misleading results.
How to test soil pH
Professional testing: RHS soil analysis service provides a full written report.
Home testing: You can test your soil pH yourself using a DIY kit widely available at garden centres. These kits are relatively cheap and easy to use and give a good indication of soil pH. But for the best results, send a soil sample to a laboratory for detailed analysis.
Always follow the sampling directions given by the test kit or laboratory to get a representative sample for the area in question.
Laboratory tests also detect free calcium carbonate (chalk or limestone). This may not be measured by DIY kits. A quick home test to check for free calcium carbonate is to add vinegar to a soil sample. If ‘fizzing’ is seen, free calcium carbonate is present.
Interpreting the results of a soil pH test
A pH test measures soil acidity or alkalinity. A pH 7.0 is considered neutral. An acid soil has a pH value below 7.0. Above pH 7.0 the soil is alkaline.
pH 3.0 - 5.0
- Very acid soil
- Most plant nutrients, particularly calcium, potassium, magnesium and copper, become more soluble under very acid conditions and are easily washed away
- Most phosphates are locked up and unavailable to plants below pH 5.1, although some acid tolerant plants can utilise aluminium phosphate
- Acid sandy soils are often deficient in trace elements
- Bacteria cannot rot organic matter below pH 4.7 resulting in fewer nutrients being available to plants
- Action: Add lime to raise the pH to above 5.0. The addition of lime can help break up acid clay soils
pH 5.1 - 6.0
- Acid soil
- Ideal for ericaceous (lime-hating) plants such as rhododendrons, camellias and heathers
- Action: Add lime if other plants are grown
pH 6.1 - 7.0
- Moderately acid soil
- A pH 6.5 is the best general purpose pH for gardens, allowing a wide range of plants to grow, except lime-hating plants
- The availability of major nutrients is at its highest and bacterial and earthworm activity is optimum at this pH
- Action: It is not usually necessary to add anything to improve soil pH at this level
pH 7.1 - 8.0
- Alkaline soil
- Phosphorus availability decreases
- Iron and manganese become less available leading to lime-induced chlorosis
- But an advantage of this pH level is that clubroot disease of cabbage family crops (brassicas) is reduced
- Action: Sulphur, iron sulphate and other acidifying agents can sometimes be added to reduce pH. Clay soils often require very large amounts of acidifying material and soils with free chalk or lime are not usually treatable
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